TDI has been developing fiber optic sensors and systems for medicine and industry since 1980; medical sensors completed (i.e., pressure, temperature) and under development (i.e., pH, pCO2, pO2) have been licensed for use in disposable intravascular catheters. These disposable smart catheters' will be manufactured and marketed by a major U.S. catheter company beginning in 1985. For the present program, TDI proposes the development of indicator chemistries specific for sodium and potassium ions. Chemistries will be selected/developed that show a specific and reversible response to these ions, and further, that cause a measurable shift is spectral energy to changes in specific ion concentrations. Additionally, TDI will modify its existing optoelectronic system design to work with these new indicator chemistries. Finally, TDI will prototype sensors and systems that in turn will allow for testing concept feasibility within this Phase I program. The emerging market for 'smart catheters' will be driven in part by demonstrated user convenience and cost effectiveness. The convenience of on-line, real-time measurement information will increase the efficiency of patient diagnosis and therapy. Further, 'smart catheters' will provide for intravascular measurements of sodium and potassium ion concentration for the price of one or two conventional in vitro laboratory assays.